Tunable spin-orbit coupling synthesized with a modulating gradient magnetic field

We report the observation of tunable spin-orbit coupling (SOC) for ultracold $^{87}$Rb atoms in hyperfine spin-1 states. Different from most earlier experiments where atomic SOC of pseudo-spin-1/2 are synthesized with Raman coupling lasers, the scheme we demonstrate employs a gradient magnetic field (GMF) with ground state atoms and is immune to atomic spontaneous emission. The effect of the SOC is confirmed through the studies of: 1) the collective dipole oscillation of an atomic condensate in a harmonic trap after the synthesized SOC is abruptly turned on; and 2) the minimum energy state at a finite adiabatically adjusted momentum when the SOC strength is slowly ramped up. The coherence properties of the spinor condensates remain very good after interacting with modulating GMFs, which prompts the enthusiastic claim that our work provides a new repertoire for synthesized gauge fields aimed at quantum simulation studies with cold atoms.